1. The Field of the Invention
This invention relates generally to fiber optic data transmission systems, and more particularly, to a bi-directional fiber optic computer link.
2. Description of the Prior Art
High speed modems such as the Astro-Jet-400 Series Data Communication Systems manufactured by Astrocom Corporation and the 303 Type Wideband Data Stations manufactured by American Telephone and Telegraph Company are well known. These prior art systems employ twisted pair and coaxial cable as a transmission medium and, as a result, have several disadvantages.
One such disadvantage resides in the susceptibility of electrical lines to electromagnetic interference which causes random burst errors to appear on the line. Thus, in order to assure data integrity, complex error correcting and detecting techniques and retransmission techniques are employed. This requirement greatly increases the cost of the system and decreases the transmission data rate.
A further disadvantage results from the requirement that a return line be provided. Since the signal is referenced to a particular ground potential, problems arise due to ground potential shifts.
Additionally, complex logic is required to compensate for the different amounts of attenuation of the various frequencies being transmitted. Further, the use of coaxial cable results in cable congestion which is a serious drawback in environments which require dense packaging of processor units.
With the vast improvements in optical fiber technology and associated source and detector components, the use of fiber optics in computing systems is very much a reality. Many of the problems associated with twisted pair and coaxial cable, briefly mentioned above, are substantially reduced or eliminated. For example, transmission through optical fibers is performed optically in accordance with principles governing dielectric waveguide propagation. Such transmission is not susceptible to electromagnetic interference. Thus, the need for elaborate error detecting and correcting capabilities is no longer necessary. Further, there need be no concern over ground potential shifts, since the two units being interconnected are completely independent electrically.
The problems associated with the attenuation along electrical conductors is substantially reduced in a fiber optic signal, since the actual signal is modulated onto light. The frequency of the signal is such a small percentage of the frequency of the light that the optical transmission characteristics have virtually no effect on the information content. In contrast, a signal transmitted over twisted pair is transmitted with no high frequency carrier and thus the effects of the transmission media are felt to a much greater extent.
Finally, there are other advantages associated with fiber optics which make it a viable form of data communication. First, there is the obvious advantage of reduced size which is important in situations where dense packaging requirements exist. Second, fiber optics offer wider bandwidth and lower attenuations. Third, much higher data rates are possible using fiber optics, and transmission can be accomplished over much longer lengths, e.g., one kilometer. Finally, problems such as signal ringing, line capacitance, and dangerous short circuiting are substantially reduced.